Cathode ray tube system including means for varying beam intensity



Sept. 2, 1969 J. E. HIGBEE ET AL 3,465,200

- CATHODE RAY TUBE SYSTEM INCLUDING MEANS FOR VARYING BEAM INTENSITYFiled Jan. 25, 1967 2 Sheets-Sheet 1 CATH. GRID vERT. HoR. 9 I DRIVEDRIVE DEFLEcT DEFLECT 24 CT. CT. MEANS MEANS 1 I 5 BEAM CONT. SI 6.

SOURCE 22 DEFLEC SIGNAL SouRcE BEAM BEAM INTENSITY INTENSITY DEFOCUSINGTHRESHOLD l s I IN (c) GA! N l G 2 I INI ENTOR.

- MARTIN c. HENDERSON ,JOHN E. HIGBEE ATTORN YS p 1969 .1. E. HIGBEE ETAL CATHODE RAY TUBE SYSTEM INCLUDING MEANS FOR VARYING BEAM INTENSITY 2Sheets-Sheet 2 Filed Jan. 25, 1967 W U 6 www RR GDC I I l l l I I I l IJ AU O 3 w T. M 6 a 5; D R IE K E C v F E A mB LD NE OE 5 M 4 5 FIGQ3INVENTOR. MARTIN C. HENDERSON JOHN E. HIGBEE BY FIG. 4

ATTORNEYS United States Patent M 3,465,200 CATHODE RAY TUBE SYSTEMINCLUDING MEANS FOR VARYING BEAM INTENSITY John E. Higbee, Santa Susana,and Martin C. Henderson,

Canoga Park, Calif., assignors to The Bunker-Ramo Corporation, CanogaPark, Calif., a corporation of Delaware Filed Jan. 23, 1967, Ser. No.610,952 Int. Cl. H013 29/52 US. Cl. 31530 8 Claims ABSTRACT OF THEDISCLOSURE Circuit apparatus useful in a cathode ray tube display systemfor facilitating accurate control of beam intensity thus enablingintensity variations resulting from variations in beam velocity to becompensated for. The nonlinearity of the beam intensity versus controlvoltage characteristic typical of most cathode ray tubes is compensatedfor by incorporating a nonlinear feedback path in the amplifierproducing the control voltage in response to an input voltage.Additionally, the amplifier incorporates a limiting feed-back path toprevent the grid voltage from increasing beyond a certain thresholdlevel at which defocussing occurs.

The invention herein described was made in the course of or under acontract or subcontract thereunder, with Department of the Air Force,Rome Air Development Center.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates generally to cathode ray tube (CRT) display systems and moreparticularly to means for use therein for facilitating the accuratecontrol of beam.

intensity.

In certain sophisticated display systems, CRTs are called upon todisplay a variety of significantly different elements or symbols alldrawn in constant time irrespective of size. For example, in such asystem the CRT may be called upon to describe both a small circle (e.g.,inch diameter), and a large circle (e.g., 6 inch diameter) in a singledisplay pattern. It the two circles are drawn in the same amount oftime, it should, of course, be apparent that the beam velocity must bemuch greater to describe the large circle than to describe the smallcircle. It should also be apparent that if the beam is moved with agreater velocity, it will appear less intense to an observer. In orderto prevent this intensity variation, prior art systems have incorporatedbeam intensity control means responsive to beam velocity. It is found,however, prior art intensity control means are inadequate to provideaccurate intensity control over a wide dynamic range because of thenonlinearities typically found in CRTs.

SUMMARY OF THE INVENTION The present invention is directed to a systemenabling CRT beam intensity to be controlled extremely accurately over awide dynamic range.

Briefly, in accordance with one aspect of the present invention, meansare provided for introducing nonlinearities between an input signal andan output control electrode (e.g., grid) signal to compensate fornonlinearities in the relationship of beam intensity to the controlsignal.

In the preferred embodiment of the present invention, the nonlinearitiesare introduced by an amplifier to which the input signal is applied. Anonlinear feedback path is employed whose impedance changes as the levelof the input signal changes. Variations in the feedback impedance, ofcourse, vary the closed loop gain of the amplifier.

Therefore, by proper selection of circuit components, beam intensity canbe made to be a substantially linear function of the input signal.

Briefly, a further aspect of the present invention is based on therecognition that beam defocussing or blooming may occur as the controlor grid signal exceeds a certain threshold level, and that byautomatically limiting the grid signal variation this defocussing can beprevented.

Thus, in accordance with a further aspect of the preferred embodiment ofthe invention, an additional amplifier feedback path is provided whichreduces the amplifier gain after the amplifier output signal exceeds acertain threshold level.

DESCRIPTION OF THE DRAWINGS FIGURE 1 is a block diagram of a CRT displaysystem which incorporates the present invention;

FIG. 2(a) illustrates a typical CRT characteristic;

FIG. 2(b) illustrates an ideal CRT characteristic;

FIG. 2(0) illustrates a gain characteristic provided by an embodiment ofthe present invention;

FIG. 3 is a block diagram of the grid drive circuit of FIG. 1 inaccordance with the present invention; and

FIG. 4 is a schematic circuit diagram of the grid drive circuit of FIG.3.

Attention is now called to FIG. 1 which illustrates a CRT display systemwhich can incorporate the teachings of the present invention. Moreparticularly, the system of FIG. 1 utilizes a CRT 10 having an electrongun including a beam producing electrode or cathode 12 and a controlelectrode or grid 14. Although other electrodes such as an anode wouldalso be incorporated within the tube 10, such other electrodes are notillustrated herein inasmuch as they do not bear upon the presentinvention. The cathode ray tube 10 also includes a face 16 coated withan illuminable material such as phosphor. In the normal operation of thetube 10, an electron beam is provided by the cathode 12 and impingesupon the face 16 to develop a light spot.

The position of the beam with respect to the face 16 is controlled by avertical deflection means 18 and a horizontal deflection means 20 whichare responsive to signals provided by a deflection signal source 22. Itwill be appreciated that by applying appropriate deflection signals tothe means 18 and 20, the beam can be caused to describe any desiredpattern on the CRT face 16.

The beam provided by cathode 12 can be selectively blanked and unblankedin response to signals provided by the beam control signal source 24 tothe cathode drive circuit 26 whose output terminal is coupled to thecathode 12. Thus, for example, the beam can be unblanked by establishinga large accelerating potential gradient between the cathode 12 and theanode (not shown). As an example, assume that the beam is unblanked byapplying a +60 volt potential to the cathode 12. On the other hand, thebeam can be blanked by raising the potential applied to the cathode 12to volts, for example, to thus reduce the accelerating potentialgradient.

Typically, in the operation of the tube 10, the beam will be moved withconsiderably different velocities in generating a display. This is sobecause in many systems each symbol or line is drawn in constant timeregardless of the size of the symbol or line. Thus, if a short vectorand a long vector are described in the same amount of time, it isapparent that the beam must be moved with a greater velocity to describethe long vector. When the beam moves with a greater velocity, the lightintensity developed thereby decreases. In other words, beam intensity isroughly inversely proportional to beam velocity. In order to compensatefor these intensity variations and thus enable all information to bedisplayed with near uniform intensity, some prior art systems haveprovided means for varying the intensity as a function of beam velocity.Beam intensity can be controlled by controlling the potential applied tothe grid 14, for example. Thus, as the potential E provided by drivecircuit 30 to the grid 14 is increased toward the potential of thecathode 12, the beam intensity will increase. However, as is illustratedby FIG. 2(a), the beam intensity versus grid potential characteristic ofa CRT is typically nonlinear. That is, at low grid potentials when thereis a large negative potential gradient from the cathode 12 to the grid14 the beam intensity varies by only a small increment in response to aunit change of the grid voltage E However, as the grid potentialincreases and the negative gradient from the cathode to the griddecreases, small changes in grid voltage E will cause much largerchanges in the beam intensity. Furthermore, as the grid voltageapproaches the cathode voltage the CRT will typically defocus seriously.

It would, of course, be ideal for the beam intensity to varysubstantially linearly as a function of the input signal E provided bythe control signal source 24 to the drive circuit 30, as is illustratedby linear portion 36 of FIG. 2(b). Additionally, it is desirable tolimit the beam intensity, as shown at 38 in FIG. 2(b), to preventdefocussing after the input voltage exceeds a certain threshold level.

In accordance with the present invention, the grid drive circuit 30 isdesignated to introduce nonlinearities in the characteristic relatingthe grid signal E to the input signal E In accordance with a preferedembodiment of the invention, th gain (E /E of the drive circuit 30 isvaried as the level of the input signal E increases. Thus, from aconsideration of FIG. 2(a), it should be apparent that a greater amountof gain is required for low levels of B with decreasing gain required assignal E increases. Thus, the preferred embodiment of the invention asillustrated in FIGS. 3 and 4 provides for different gain levels whichdecrease as the signal E increases. More particularly, at a low level ofsignal E a relatively high gain 40 is exhibited by the drive circuit 30.As the level of signal E increases, the gain is successively decreasedto level 42, level 44, and level 46. It will, of course, be appreciatedthat the gain characteristic illustrated in FIG. 2(c) is exemplary onlyand that the gain characteristic should be selected to approximate thelinear portion 36 of the ideal characteristic of FIG. 2(b). It shouldfurther be appreciated that it is not at all necessary to actuallyachieve the ideal characteristic of FIG. 2(b) inasmuch as small amountsof beam intensity variation will be barely noticed by a system user andin any event will not significantly detract from the clarity or estheticcharacter of the display.

Attention is now called to FIG. 3 which illustrates a block diagram ofthe grid drive circuit 30 of FIG. 1 which includes means for bothintroducing nonlinearities to linearize the relationship between beamintensity and input signal E and means for limiting the beam intensityas the signal E exceeds a threshold level. Briefly, the drive circuit 30is preferably comprised of two or more amplification stages 50 and 52.The signal E is applied through impedance 54 to the input terminal ofamplifier 50. The output terminal of amplifier 50 is coupled to theinput terminal of amplifier 52. The grid signal E; is provided on theoutput terminal of amplifier 52.

In order to introduce the nonlinearities previously discussed, anonlinear feedback path 56 is connected between the output terminal ofamplifier 52 and the input terminal of amplifier 50. The impedance ofthe nonlinear feedback path 56 varies as the input signal E varies tothus vary the gain exhibited to an input signal as illustrated by FIG.2(c).

In order to limit beam intensity as shown by the porttion 38 of FIG.2(b), a limit feedback path 60 is provided between the output and inputterminals of amplifier 52. The limit feedback path 60 is normally open.However, when the output signal provided by amplifier 52 tries to exceedan established threshold level, the feed back path 60 is closed to thusreduce the closed loop gain of the amplifier 52 to a very small value.Thus, the output signal E provided by amplifier 52 will be maintained atthe threshold level, thereby preventing the beam defocussing previouslymentioned.

Attention is now called to FIG. 4 which illustrates the grid drivecircuit 30 of FIG. 3 in greater detail. More particularly, as can beseen from FIG. 4, the nonlinear feedback path 56 is comprised of afeedback resistor R connected between the output of amplifier 52 and theinput of amplifier 50. A plurality of circuit paths are connected inparallel with the feedback resistor R Each of these parallel pathsincludes a diode which is normally back biased but which becomes forwardbiased as the input signal E increases to thus gradually insertresistance in parallel with the resistor R to thus reduce gain. It willbe recalled that the closed loop gain of an amplifier having a feedbackloop coupled from its output to its input is approximately equal to theratio between the effective feedback resistance and the inputresistance, herein resistor 54. Thus, if the feedback resistance can bereduced by gradually introducing resistance in parallel with resistor Rthe gain can be correspondingly reduced.

More particularly, a first path in parallel with the feedback resistanceR is comprised of resistor R connected in series with resistor R and aswitch, preferably a diode D A second path in parallel with feedbackresistor R includes resistor R connected in series with resistor R anddiode D A third parallel path is comprised of previously mentionedresistor R connected in series with resistors R R and diode D Resistor Rconnects the junction between resistors R and R to a source of negativepotential. A variable resistor R similarly connects the junction betweenresistors R and R to the source of negative potential.

In the operation of the feedback path 56 for low levels of input signalE each of the diodes D D and D will be back biased so that the feedbackresistance will be defined solely by the resistor R Thus, the gain ofthe drive circuit of FIG. 4 will be defined by the ratio between thevalue of resistor R and the value of input resistor 54. As the level ofthe input signal E increases, the output signal E also increases, thusincreasing the potential at the junction between resistor R and variableresistor R When the level of E increases sufiiciently, diode D will beforward biased to thus insert resistors R and R in parallel withfeedback resistor R to accordingly lower the effective feedbackresistance and correspondingly lower the closed loop gain. It will beappreciated that the level of signal E required to forward bias thediode D depends upon the setting of variable resistor R As the inputsignal level increases further, the diode D and subsequently the diode Dwill become forward biased to thereby introduce additional resistance inparallel with the feedback resistor R to further lower the effectivefeedback resistance and the closed loop gain.

Accordingly, it should now be appreciated that the gain level 40represented in FIG. 2(c) is established when all the diodes D D and Dare back biased. When diode D becomes forward biased, the closed loopgain decreases to the level 42 and as diodes D and D become successivelyforward biased the gain decreases further to level 44 and subsequentlyto level 46.

The limit feedback path 60 includes a diode'D connected in series with aresistor R to the emitter of PNP transistor Q1. The collector of thetransistor Q1 is connected to the input terminal of amplifier 52. Thebase of transistor Q1 is connected to the tap 61 of a potentiometer 62connected between ground and a source of positive potential.

The tap 61 of potentiometer 62 is utilized to establish a referencepotential on the base of transistor Q1. As long as the output signal Eprovided by the amplifier 52 is not greater than the potential appliedto the base of transistor Q1, the diode D will be back biased and thetransistor Q1 will be held off. On the other hand, when the signal Eincreases to a level two diode drops greater than the potentialestablished on the potentiometer tap 61, the diode D will become forwardbiased and turn the transistor Q1 on. The introduction of the lowimpedance feedback loop around amplifier 52 through transistor Q1reduces the closed loop gain of the amplifier 52 to a very small valuesuch that the level of signal E cannot rise much above the potentialestablished by the potentiometer 62.

It is pointed out that the feedback path 60 is connected aroundamplifier stage 52 rather than around both stages 50 and 52 primarily toassure stability. That is, inasmuch as the feedback through path 60 isaround fewer stages than the principal signal loop through path 56, itwill introduce less phase shift and therefore will not tend tooscillate.

From the foregoing, it should be appreciated that a circuit apparatushas been disclosed herein for enabling beam intensity in a cathode raytube to be controlled extremely accurately over a wide dynamic range.Accurate control is achieved by introducing nonlinearities in the gridelectrode drive circuit which nonlinearities tend to linearize therelationship between beam intensity and input signal E Additionally, thedisclosed circuit apparatus introduces limiting means for preventing thegrid signal E from increasing beyond a certain level at which severebeam defocussing can occur.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In combination with a cathode ray tube having beam producing meansand a control electrode, drive circuit means responsive to an inputsignal for providing a control signal to said control electrode forcontrolling the intensity of said beam, said cathode ray tube having anonlinear characteristic relating beam intensity to control signallevel, said drive circuit means including:

an amplifier having input and output terminals;

means for applying said input signal to said amplifier input terminal;

means coupled to said amplifier for nonlinearly varying the gain thereofas a function of input signal level to substantially linearize thecharacteristic relating beam intensity to input signal level;

means coupling the output terminal of said amplifier to said controlelectrode;

means including a normally open feedback path connected between saidoutput terminal and said input terminal for limiting said control signalto a predetermined level; and

means for closing said normally open feedback path in response to saidcontrol signal reaching said predetermined level.

2. The combination of claim 1 wherein said normally open feedback pathincludes a transistor having an emitter, a collector, and a base;

means connecting said emitter and collector between said amplifier inputand output terminals; and

means applying a variable potential to said base.

3. The combination of claim 1 wherein said means for varying gainincludes a second feedback path including a feedback impedance couplingsaid amplifier output terminal to said amplifier input terminal; and

means for varying said feedback impedance as a function of input signallevel.

4. The combination of claim 3 wherein said feedback impedance includes afixed resistor; and wherein said means for varying said feedbackimpedance includes a plurality of impedance paths connected in parallelWith said feedback resistor;

a plurality of switch means, each connected in a different one of saidpaths; and

means for closing each of said switch means in response to a differentlevel of control signal.

5. The combination of claim 4 wherein each of said switch meanscomprises a diode.

6. In combination with a cathode ray tube having beam producing meansand a control electrode, drive circuit means responsive to an inputsignal for producing a control signal for application to said controlelectrode for controlling the intensity of said beam, said cathode raytube having a nonlinear characteristic relating beam intensity tocontrol signal level, said drive circuit means includng:

an amplifier having at least first and second stages, each stage havinginput and output terminals;

means for applying said input signal to said input terminal of saidfirst stage;

means coupling said first stage output terminal to said second stageinput terminal;

means coupling said second stage output terminal to said controlelectrode;

first feedback means coupled between said second stage output terminaland said first stage input terminal for nonlinearly varying the gain ofsaid amplifier as a function of input signal level to substantiallylinearize the characteristic relating beam intensity to input signallevel; and

second feedback means connected between said second stage outputterminal and said second stage input terminal for limiting said controlsignal to a predetermined level.

7. The combination of claim 6 wherein said first feedback means includesa feedback impedance; and

means for varying said feedback means as a function of input signallevel.

8. The combination of claim 6 wherein said second feedback meanscomprises a normally open feedback path 1ncluding a transistor having anemitter, a collector, and a base;

means connecting said emitter and collector between said second stageoutput terminal and said second stage input terminal; and

means coupling a controllable potential source to said transistor base.

References Cited UNITED STATES PATENTS 2,218,720 10/1940 Rinia 315-302,240,289 4/1941 Dillenburger et a1. 315-30 2,567,377 9/1951 Holbrook315-30 2,671,871 3/1954 Haynes 315-30 2,860,284 11/1958 McKim 315-30X3,277,335 10/ 1966 Moser et al 315-30 RODNEY D. BENNETT, 111., PrimaryExaminer M. F. HUBLER, Assistant Examiner

